The subject matter of the present innvention relates generally to pyrolysis reaction apparatus and methods in which carbonizable waste material is heated in a reaction chamber in the absence of oxygen to decompose the material by pyrolytic reaction and produce hydrocarbon gas, liquid and solid residue. Such pyrolysis apparatus and method are particularly adapted for the treatment of material containing precious metal, such as photographic film, to recover silver or other precious metal without undue loss of the metal while operating in an efficient, inexpensive and trouble-free manner without polluting the environment.
With the pyrolysis apparatus and method of the present invention the carbonizable material is fed into the reaction chamber through a water cooled inlet conduit which passes through the wall of the furnace surrounding such reaction chamber. As a result of feeding it into a high temperature inlet portion of the reaction chamber, the material is immediately heated above its vaporization temperature in the reactor chamber. By cooling the inlet conduit with a cooling fluid jacket, such material is maintained below its melting temperature in the inlet conduit to prevent clogging of such conduit. The material is fed into the reaction chamber along an input feed path which is laterally offset from the axis of rotation of a auger type impeller means conveying the material within such chamber and is directed toward the tips of the impeller blades in order to prevent clogging of the impeller.
The pyrolysis reaction apparatus and method of the present invention is especially useful in the recovery of silver from the silver halide in photographic film including used X-ray film, without undue loss of the silver such as in conbustion vapor. The film is comminuted into film particles that are fed into the pyrolysis reaction chamber where they are vaporized to produce a combustible hydrocarbon pyrolysis gas and a solid carbonized residue containing silver which are both discharged from the reaction chamber through separate outlets. The pyrolysis gas may be fed to a burner within the furnace for combustion to heat the reaction chamber. The solid residue is reduced in size and weight by a factor of about 10 to 1 compared to the input material. Such residue is oxidized to remove the carbon and to produce silver oxide which is refined to recover the silver.
The present pyrolysis method of recovering silver from photographic film has many advantages over previous recovery methods. The most common recovery method is to burn the film in air which looses up to 40% of the silver in combustion vapor as well as creating an air pollution problem. Another method involves chemical treatment of the film for removing the silver by chemical reaction without burning so that the higher percentage of silver is recovered without an air pollution problem. However, such chemical treatment is extremely expensive and therefore impractical. These problems are avoided by using a pyrolysis process to recover silver from photographic film, but previous attempts to use pyrolysis were not successful because of clogging problems. Thus, clogging of carbonizable deposits were formed on the reaction chamber impeller, the inlet conduit and the outlet conduit of such reaction chamber because the film particles were fed along an inlet path toward the impeller axis and the film first melted in the reaction chamber before vaporizing. Such clogging also caused air to be sucked into the reaction chamber and such conduits causing explosions.
The pyrolysis apparatus and method of the present invention is an improvement over such prior pyrolysis apparatus and methods because with such invention the material to be treated is fed into the reation chamber by an inlet conduit which extends through the furnace wall and enters such chamber at an inlet region of high temperature for immediate vaporization of such material. In order to prevent melting of the material within the inlet conduit in the present invention, such conduit is surrounded with a fluid cooling jacket which maintains the temperature within the inlet conduit below the melting temperature of such material. In such prior pyrolysis apparatus and methods and those of U.S. Pat. No. 4,123,332 of rotter issued Oct. 31, 1978, U.S. Pat. No. 1,972,929 of Fisher issued Sept. 11, 1934 and U.S. Pat. No. 1,461,614 of Harrison issued July 10, 1923, the material to be treated is fed into the reaction chamber through an inlet conduit entering such chamber outside of the furnace. As a result the material enters a relatively cold inlet region of the reaction chamber where the material does not immediately vaporize, but first melts so that it clogs the inlet conduit as well as the auger impeller means used to convey the material through the reactor chamber. This clogging problem can be extremely dangerous because it can cause air to be sucked into the reaction chamber and conduits which may cause an explosion if the reaction chamber and inlet and outlet conduits are not cleaned frequently on a regular basis. Such regular cleaning necessitates shutting down of the pyrolysis apparatus and removal of the clogging deposit is difficult and time consuming so that the apparatus is very expensive to operate. Cooling of the impeller means within the reaction chamber by transmitting cooling fluid through the hollow impeller shaft in order to prevent melting of the impeller blades or deflection of the impeller shaft is shown in the above cited references as well as in the coal gasification apparatus of U.S. Pat. No. 2,983,653 of Danualt et al. issued May 9, 1961. However, this may be undesirable because too much cooling of the impeller can cause clogging deposits to condense on the impeller shaft and blades. This latter patent does not employ a furnace but the coal or other material to be treated is mixed with solid particles of heat carrier material such as sand or coke which is injected into the mixing chamber for heating such material. Also the mixing chamber of Danualt is not sealed to prevent the entry of oxygen containing gas so that it is not a pyrolysis reaction chamber. Previously pyrolysis reaction apparatus and methods have been used to convert organic waste into fuel including hydrocarbon gas which have been used to fuel a burner used in the furnace surrounding the reaction chamber as discussed in U.S. Pat. No. 4,123,332 of Rotter and in U.S. Pat. No. 4,235,676 of Chambers issued Nov. 25, 1982. However, none of these patents show the above-discussed deficiencies of the earlier discussed references.